This application claims the priority benefit of Taiwan application serial no. 90127128, filed Nov. 1, 2001.
1. Field of Invention
The present invention relates to a type of display device. More particularly, the present invention relates to a storage capacitor structure of a display device.
2. Description of Related Art
Display devices have found widespread usage in our daily life. Television and computer monitors are common display devices that show different kinds of images or motions on a screen. Formerly, cathode ray tubes were widely used. However, due to bulkiness and power consumption, cathode ray tubes cannot be used for portable equipment such as a notebook computer. Nowadays, consumers welcome the newly developed dot matrix type of flat panel displays such as liquid crystal display (LCD) or thin film transistor (TFT) LCD. An array of picture pieces or pixels on the TFT LCD constitutes an image with the switching of each pixel controlled by a thin film transistor.
FIG. 1 is a schematic diagram showing the driving circuit of a conventional thin film transistor liquid crystal display. The TFT LCD requires a scan circuit 100 and a signal-holding circuit 102. The scan circuit 100 drives a group of scan lines 110 and the signal-holding circuit 102 drives a group of signal lines 112. The scan lines 110 and the signal lines 112 cross each other perpendicularly forming a two-dimensional array. Each cross-point in the two-dimensional array has a thin film transistor 104, a storage capacitor 108 and a liquid crystal display (LCD) cell 106. The thin film transistor 104, the storage capacitor 108 and the LCD cell 106 together constitute a pixel. The gate terminal of the thin film transistor 104 is controlled by the corresponding scan line 110 and the source terminal of the thin film transistor 104 is controlled by the corresponding signal line 112. The drain terminal of the thin film transistor 104 is connected to a pixel electrode layer and an electrode of the storage capacitor 108. The storage capacitor 108 maintains a voltage for controlling the liquid crystals. Another electrode of the storage capacitor 108 is connected to an adjacent scan line.
Following the gradual reduction in dimensional layout of a thin film transistor, a common electrode type of storage capacitor design is selected for reducing the effect of gate-driven delay. In this design, the common electrode and the gate terminal are separated from each other so that the other terminal of the capacitor receives a common voltage such as a common electrode voltage (Vcom).
FIG. 2A is a schematic layout diagram of a unit cell of a conventional thin film transistor liquid crystal display device. As shown in FIG. 2A, the gate terminal of the thin film transistor 104 is connected to the scan line 110. The source terminal of the thin film transistor 104 is connected to the corresponding signal line 112. The drain terminal of the thin film transistor 104 is connected to a pixel electrode layer 118. A lower electrode 114 and an upper electrode 116 together constitute a storage capacitor. The pixel electrode layer 118 and the upper electrode 116 are linked through an opening 120.
FIG. 2B is a diagram showing a cross-sectional view along line Ixe2x80x94I of FIG. 2A. As shown in FIG. 2B, a first metallic layer is formed over a transparent substrate 126. The lower electrode 114 together with the gate terminal of the thin film transistor 104 are formed by patterning the metallic layer. A capacitor dielectric layer 124 is formed over the lower electrode 114. A metallic electrode layer 116 is formed over the capacitor dielectric layer 124 to serve as the upper electrode of the storage capacitor. The overlapping region between the upper electrode 116 and the lower electrode 114 is the main charge storage area for the capacitor. A passivation layer 122 is formed over the upper electrode 116 and surrounding areas. The passivation layer 122 has an opening 120 that exposes a portion of the upper electrode 116. A pixel electrode layer 118 is electrically connected to the upper electrode 116 through the opening 120.
In the aforementioned LCD structure, if some extrinsic residual material 115 remains somewhere in the neighborhood of the capacitor, especially near the edge of the lower electrode 114, a short-circuit path may form. Hence, the storage capacitor 108 may lose its function leading to point defect in the pixel. The extrinsic residual material 115 may be removed by shining a laser beam. However, the process may also break the normal line connection with the common electrode 114 and lead to a shallow line for the gate terminal. To prevent the formation of shallow lines, most design engineers prefer not to repair the defective capacitor and leave the bright spot as it is.
Nevertheless, stringent demand for high quality image in the market is a major force for the use of laser to repair bright spot and attain a zero bright spot target. At present, laser repair technique has not progressed far enough for spot darkening to be carried out as routine. This is because the common electrode and the gate terminal may form a short circuit after the repair resulting in a bright line defect. Thus, any method capable of repairing storage capacitor point defect and at the same time permitting the execution of spot darkening operations is needed for improving image quality.
Accordingly, one object of the present invention is to provide a storage capacitor structure such that a pixel electrode and an electrode of the capacitor can be detached by cutting if the two electrodes of the capacitor form a short circuit. In this way, the pixel electrode serves as an electrode for the capacitor so that the overlapping portion between the pixel electrode and the capacitor electrode still constitute a storage capacitor.
To achieve this and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a storage capacitor structure. The storage capacitor structure includes a first capacitor electrode over a substrate. A capacitor dielectric layer is formed over the first capacitor electrode. A second capacitor electrode is formed over the capacitor dielectric layer. A passivation layer is formed over the second capacitor electrode. The passivation layer has an opening that exposes a portion of the second capacitor electrode. A pixel electrode layer is formed over the passivation layer such that the pixel electrode layer has a protruding section for connecting with the second capacitor electrode through the opening in the passivation layer. When the first capacitor electrode and the second capacitor electrode form a short circuit, the protruding section may be cut to separate out the second capacitor electrode.
In the aforementioned storage capacitor structure, if the first capacitor electrode and the second capacitor electrode are in short circuit, the protruding section is cut to detach it from the second capacitor electrode. Thereafter, the pixel electrode layer also serves as an upper electrode of the storage capacitor.
In the aforementioned storage capacitor structure, the protruding section of the pixel electrode layer further includes a neck section and a connective section. The neck section is a removable section that can be cut for detachment, while the connective section and the second capacitor electrode are joined together.
In the aforementioned storage capacitor structure, the pixel electrode layer lies outside the protruding section. However, the pixel electrode layer also overlaps with a portion of the second capacitor electrode and the first capacitor electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.